The present invention relates to a modulator circuit for a common gate silicon carbide (SiC) transistor amplifier. More specifically, the present invention relates to an adjustable, pulsed, feedback-controlled bias voltage supply for a common gate SiC transistor amplifier for providing increased efficiency and lower power dissipation as compared to traditional shunt-to-ground source regulators known heretofore.
FIG. 1-A shows a conventional configuration for biasing a static induction transistor (SIT) 12, typically a SiC transistor, in a common gate configuration. A relatively high voltage power supply 20 is connected between the drain and ground through a coil RFC-1, usually with a switch 21, which pulse enables the SIT device 12. Normal operation requires the gate to be biased negative with respect to the source, and this is accomplished with a separate low voltage supply 18, nominally of about 9 volts in series with RF choke coil RFC-2. Because different transistors typically require slightly different source to gate bias for optimum operation, the low voltage output of source 18 is adjustable, usually from 7 to 9 volts. It can be seen that the drain current ID comes from the high voltage supply 20, but it also flows through the source supply 18 dissipating considerable power in it. The drain to source voltage is only 81 volts, but the current is supplied from the 90 volt high voltage supply 20. The power dissipated in the 9-volt source supply 18 is thus wasted. In high power modules, multiple transistors are connected in parallel, with a common high voltage power supply, and individually adjustable source biases.
The problems with the prior art are:
1. Wasted power in the source bias supply;
2. A high voltage drain switch, which is bulky and slow.
Accordingly, it is a primary aspect of the present invention to provide a circuit and method for minimizing power dissipation by the source power supply in a static induction transistor operating in a common gate circuit configuration.
It is a further aspect of the present invention to provide a modulator for a static induction transistor amplifier which is compact and rapidly switchable between operating states.
It is still another object of the present invention to provide an electronically adjustable source bias voltage for a common gate static induction transistor.
The aspects of the present invention are fulfilled by providing a source modulator circuit for a static induction transistor (SIT) amplifier including a silicon carbide (SiC) transistor having a grounded gate, a source and a drain comprising: a source voltage supply connected between the source of the transistor and ground (gate); a drain voltage supply connected between the source and drain; and a circuit for directing current from the drain voltage supply around the source voltage supply to said source, whereby power dissipation by the source voltage supply is minimized. The current directing circuit includes a source modulator circuit including a semiconductor switch which is responsive to a control signal for turning on quiescent current in the SIT. The semiconductor switch in a preferred embodiment comprises a Field Effect Transistor (FET).
The source voltage supply may be clamped to the source of the SIT by a zener diode. Switching performance and adjustability of the SIT may also be enhanced by providing a pair of series-connected diodes between the source of SiC transistor and the drain of the FET.
Further scope of applicability of the present invention will become apparent from the detailed description provided hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.